The present invention relates to an ultrasonic apparatus for transmitting ultrasonic waves to an examination subject and receiving reflection waves from an examination subject for display and, more particularly, to the diagnosis and operation of a blood vessel with a catheter in a two-dimensional aspect of an imaging technique, and to an imaging method and probes which enable three-dimensional scanning for three-dimensional imaging or the like in a three-dimensional aspect of the imaging technique.
In a conventional two-dimensional imaging techniques, an intravascular ultrasound imaging system or a blood flow measuring apparatus of Doppler effect, can be used as an ultrasonic apparatus in which ultrasonic transducers are attached to a catheter. In most kinds of such intravascular ultrasound imaging system, transducers or ultrasonic reflectors are rotated to obtain a two-dimensional tomogram image. In some apparatus, transducers are arranged on the periphery of a catheter. A laser is inserted in a catheter for treatment of a thrombus adhered to the inner wall of a blood vessel, for example, an atheroma. For the blood flow measuring apparatus, a single plate is mounted on the catheter.
In order to scan a beam in three dimensions, there has conventionally been a structure in which two-dimensional array probes juxtaposed in directions X and Y are used, a signal line is led from each element, and delay lines are provided on almost all the transducers to form a beam either in the direction X or in the direction Y, as disclosed in Japanese Patent Unexamined Publication No. 62-4988. As a representative two-dimensional array probe, there is a probe of divided PZT, as disclosed in, for example, "Fundamental Experiment on Matrix-Array Transducer" in a collection of lecture theses of Japanese Journal of Medical Ultrasonics Supplement II (Proceeding of the 47th Meeting, November, 1985). This is a probe divided in two dimensions by grooves in matrix forms.
The inventors of the present application previously suggested a transmitter which transmits ultrasonic waves in a direction corresponding to a drive frequency in order to simplify the phasing unit, as disclosed in "High-Speed Ultrasonic Imaging System" in a collection of lecture theses of Japanese Journal of Medical Ultrasonic Supplement II (proceeding of the 36th Meeting, November, 1971).
In this transmitter, as shown in FIG. 23A, polarization-inverted array transducers are driven by frequency sweepage so as to radiate ultrasonic waves in a direction corresponding to the frequency signal. FIG. 23B illustrates a relation between an angle .theta. of radiation of ultrasonic waves and a drive signal frequency f. In FIG. 23C, the numerals 1, 2, 3, . . . n denote transducer elements formed in such a matter that the axial directions of the polarization thereof are alternately different from each other.
When driving the transducer elements by a drive frequency f.sub.1, the sonic phases of the adjacent transducer elements such as 1 and 2 are inverted relative to one another. Thus, the phase of the sonic wave (indicated by a solid line) from the transducer element 1 and the phase of the sonic wave (indicated by a broken line) from the transducer element 2 are inverted. Therefore, wave surfaces comprising sonic waves from n transducer elements are formed in a direction of .theta. to a normal direction to the transducer element surface.
When driving the transducer elements by a drive frequency f.sub.2 smaller than the drive frequency f.sub.1, the wave surfaces are formed in a different direction from the direction of .theta.. PA1 Accordingly, the ultrasonic waves can be transmitted in respective directions determined by the plural drive frequencies. When a polarization inversion pitch of the transducers is expressed by d, the drive signal frequency is expressed by f, and its wavelength is expressed by .lambda., the radiation angle .theta. can be obtained from the following equation: EQU .theta.=sin.sup.-1 (.lambda./2d) PA1 n denotes number of elements in the array PA1 .zeta.=.pi.: a phase difference between sound waves radiated from adjacent elements
Further, a far field directivity R(.theta.) at the time can be obtained from the following equation: EQU R(.theta.)=sin{n(.zeta.-.gamma.)/2}/sin{(.zeta.-.gamma.)/2} EQU .gamma.=(2.pi. d/.lambda.)sin .theta.
wherein
This relation is utilized to scan the beam.
The above-mentioned catheter in the conventional technique requires a rotational shaft for rotating the transducers or ultrasonic reflectors to obtain two-dimensional tomogram images. However, torsion of this shaft and non-uniformity in the rotations degrade the quality of images, and further, there are safety problems. Moreover, in the catheter for laser operation, an optical fiber is employed for monitoring the inside of a blood vessel, and consequently, it is necessary to displace the blood with physiological saline. The catheter with arrayed transducers has a problem that the diameter of the catheter is increased because signal lines connected to the respective transducers are extended within the catheter. In relation to the transmitter shown in FIGS. 23A, 23B and 23C suggested by the inventors of the present application, only the basic matters in the C-mode imaging are described, and no explanation has been given to tomography and application to catheters.
Furthermore, in the conventional technique, when two-dimensional arrays are used for scanning the beam in three dimensions, for example, a number n.times.n of signal lines are necessary in the case of n.times.n matrix, and in accordance with an increase in the number of the signal lines, the number of the phasing units are increased. Therefore, there is a problem that the scale of the apparatus is increased in actual construction. In the conventional technique shown in FIG. 23A, two-dimensional arrangement of the transmitter, which transmits ultrasonic waves in a direction corresponding to a drive signal frequency, has not been explained, and no method of three-dimensional beam scanning has been mentioned.